Many contemporary engine controls have integral misfire detection systems. With ever-increasingly more stringent legislated emissions standards, the assurance of accurate and complete misfire detection under all engine and vehicular operating conditions is becoming mandatory.
Commonly, system designers rely on measurement of engine acceleration, dependent largely on engine torque produced (or not produced) during a combustion process to determine misfiring of a particular engine cylinder. Given the acceleration information, misfires are predicted by various signature analysis, and/or spectral analysis, methods.
As a practical matter, an engine's acceleration behavior is also affected by powertrain related behaviors other than firing torque. These other behaviors can significantly reduce fidelity or signal-noise ratio of the primarily firing torque related acceleration signal under analysis. Furthermore, under some engine operating conditions, the noise exceeds the primarily engine torque related acceleration signal under analysis. Moreover, the noise related behavior is not limited to engine operation only causes, but include behaviors related to the complete driveline. Some noise related behaviors that are detrimental include relatively low frequency, or firing rate, driveline resonance effects, or vibrations, excited at least partially by cylinder misfiring, torque converter lockup, low speed lugging behavior characteristic of a manual transmission, a change in transmission gears and rough road conditions. Each of these, and other sources of stimulus, excite the driveline to perturbate, or transiently oscillate, at its resonant frequency.
When the above-mentioned driveline behaviors manifest themselves, and the driveline oscillates, a significant measure of what amounts to noise, relative to the misfire induced behavior, is introduced into the acceleration measurement. This noise can largely swamp out any signatory behavior of a misfire event--particularly with a non-compliant mechanical coupling between the engine and its transmission.
FIG. 1 shows a first portion 101 of a noise-free waveform indicative of an acceleration signal derived from an engine's crankshaft due to a properly firing cylinder, firing in a sequence of several cylinders, and a second portion 103 of the waveform indicative of acceleration of an engine's crankshaft due to a misfiring cylinder later in the sequence of firing cylinders. At reference number 103 the engine's crankshaft grossly decels because proper firing did not occur. Given this observation of acceleration behavior, a magnitude comparison process can monitor the engine's acceleration behavior at a predetermined threshold 105 and indicate a misfiring condition if the acceleration signal transitions below the threshold 105.
FIG. 2 illustrates a behavior of an actual acceleration signal 201 derived from a running engine over about 150 cylinder combustion cycles. This acceleration signal 201 includes a repetitively induced misfire by periodically removing a spark signal from one cylinder. From FIG. 2 it can be seen that in a real-world application, the signal derived from a running engine is effected by causes other than combustion related torque as asserted earlier. For reference purposes, the reference markers associated with the horizontal axis 203 demarcate the repetitively induced occurrences of misfire. The waveform 201 is derived using an acceleration sensing device coupled to the engine's crankshaft. Because of crankshaft torsional vibrations, inertial torque due to reciprocating masses, driveline resonance effects, and other mechanically induced vibrations on the engine's crankshaft, the waveform shown in FIG. 2 has relatively poor fidelity. This makes detection of misfire by a simple threshold detection scheme substantially hopeless.
To address the driveline ringing effects some misfire determination schemes use running average filters and/or median filters to eliminate low frequency behavior--such as driveline vibration behavior in an acceleration signal. Median filter based schemes suffer from a fixed signal-noise ratio and separation factor for any given window size. The signal-noise ratio is defined as a ratio of the peak amplitude of the signal to the peak amplitude of the noise. The separation factor can be expressed as the ratio of the difference of mean normal and misfiring acceleration with respect to the sum of the standard deviations of normal firing and misfiring acceleration. Running average filters are somewhat adequate for smoothing random nonimpulsive perturbations in the incoming signal but tend to smear sharp monotonic edge transitions that occur due to driveline inputs, whereas median filters tend to preserve the sharp driveline edge transitions while rejecting impulsive inputs (e.g., misfire acceleration behavior) but are more influenced by nonimpulsive variations.
What is needed is an improved approach for misfire detection particularly one that is insensitive to adverse powertrain operating effects. In particular, an improved system needs to account for mechanically induced vibrations on the engine's crankshaft, and driveline perturbations over a wide range of engine operating conditions. The improved technique ideally would reject both impulsive and nonimpulsive acceleration behavior to allow the underlying driveline vibration signal to be extracted from the incoming acceleration signal with high fidelity. This improved technique also needs to improve acceleration signal fidelity by improving the acceleration signal's signal-noise ratio and separation factor in order to accurately detect misfire.